Plasma-enhanced chemical-vapor deposition systems are commonly used to deposit aluminum oxide (AlOx) on silicon wafers in the production of silicon solar cells. This study investigates the impact of the deposition conditions on the obtained surface passivation quality of p-type wafers. It is the first to investigate the impact of all the five main process parameters. The ratio between the microwave power and the total gas flow rate is identified as a critical condition to form thermally stable layers. We find that the most significant parameter for high quality surface passivation is the flow rate ratio of nitrous oxide (N2O) to tri-methyl-aluminum (TMA). Higher flow rate ratio ([N2O]/[TMA]) is required to achieve better passivation for fired wafers, whereas lower ratio is preferred when the firing process is not permissible. Elastic-recoil detection analysis reveals that the gas flow rate ratio has a significant impact on the likely direction of hydrogen released from the layer during firing (either to the interface with the silicon wafer or to the environment). Surprisingly, the atomic concentration of aluminum and oxygen is found to be almost stoichiometric regardless of the wide range of the gas flow rate ratios studied in this paper.